In wired communication networks, terminal devices located in the premises of a customer (also referred to as subscriber) are connected with a central office via communication lines. An example for such a communication network is a public switched telephone network (PSTN). The corresponding telephone service is referred to as plain old telephone service (POTS). In such a case, terminal devices like telephones or facsimile devices, and also data transfer devices are connected with the central office via a pair of copper lines. The copper lines are commonly referred to as tip line and ring line. In the central office, the copper lines are typically connected with a subscriber line interface circuit (SLIC) on a corresponding line card, which in this case is referred to as a PSTN (or POTS) line card.
In the central office, a line testing procedure is typically regularly performed, to detect errors in the lines (e.g., broken lines) and also to identify if a telephone, a fax, or any other terminal device is connected to the line.
A further error or fault which may occur with such communication lines is the accidental connection of such a communication line with a line of another copper pair or other communication line, which may happen for example when communication lines are buried in the ground or when coupling connection lines in some switching device. Such faults are also referred to as a connection of the communication line to a foreign voltage, because in use a certain voltage is typically applied to communication lines.
Such a connection to a foreign voltage is often referred to as “(low) resistance to battery” or “(low) insulating resistance to battery,” because in this case a low ohmic equivalent resistance connects the wire or communication line with a different wire on which a battery voltage is typically applied (e.g., 48 V is typical in PSTN networks).
As explained above, such a connection of a communication line to a foreign voltage may occur at numerous different locations between and including the central office and the customer's premises. Consequently, there is a need for a method and an apparatus for line testing wherein the location of such a fault can be determined as precisely as possible.
Furthermore, in general line testing is typically performed with dedicated line testing equipment which is coupled to the communication lines to be tested. Such dedicated line testing equipment requires space. Furthermore, since often one such dedicated line testing equipment is used for a large number of lines, line testing typically is only performed at larger intervals, for example every seven days. Finally, such dedicated testing equipment is connected via switching elements (e.g., relays) to the communication line, which result in additional costs.
For these and other reasons, there is a need for the present invention.